Catalytically-cracked gasoline contains 20-40 vol % olefins and is therefore an important gasoline blendstock with a high octane value and a high blending ratio into finished gasoline. Catalytically-cracked gasoline is produced by catalytic cracking of heavy petroleums such as vacuum gas oil or atmospheric residue with a fluidized catalytic cracker (FCC). The sulfur content of these heavy petroleums undergoes various reactions in the production process, becoming lighter oils, and therefore sulfur compounds are present in the catalytically-cracked gasoline. In order to minimize the sulfur content of catalytically-cracked gasoline, it is common for the feed oil such as vacuum gas oil or atmospheric residue to be used in catalytic cracking after hydrodesulfurization. Heavy oil hydrodesulfurizers are high temperature-high pressure apparatuses, and the start-up costs, expansions and upgrades for such equipment needed to meet tighter restrictions on sulfur content, in line with environmental policy, lead to increased cost for both installation and operation, thus increasing the economic burden.
On the other hand, since the sulfur compounds in catalytically-cracked gasoline can be hydrodesulfurized with relatively low temperature and low pressure apparatuses, direct hydrodesulfurization of catalytically-cracked gasoline not only lowers cost for equipment investment but can also reduce operating costs compared to hydrodesulfurization of heavy oil. Nevertheless, the prior art, that is, hydrodesulfurization of catalytically-cracked gasoline in hydrodesulfurizers for naphtha, has been problematic due to hydrogenation of olefins in the catalytically-cracked gasoline which reduces the octane value. Several technologies have been proposed to solve this problem, whereby hydrodesulfurization is accomplished while limiting reduction in the octane value of catalytically-cracked gasoline. For example, there have been proposed a technique involving separation of feed oil into light and heavy components by distillation and separate hydrodesulfurization of the components under separate conditions (see Patent document 1, for example), a method of using a catalyst with controlled molybdenum and cobalt loading weights and support surface areas (see Patent document 2, for example), a method of combination with a zeolite catalyst to prevent reduction in octane value (see Patent document 3, for example), and a method using a catalyst subjected to specific pretreatment (see Patent document 4, for example). Among processes for producing gasoline with low sulfur contents there has been proposed a process for producing gasoline that includes a step of hydrogenation of the unsaturated sulfur-containing compounds and a step of decomposition of the saturated sulfur-containing compounds (see Patent document 5, for example). Such processes, however, are suitable for treatment of catalytically-cracked gasoline with high sulfur content but not for production of gasoline with very low sulfur content.
The need for “sulfur-free gasoline” with even lower sulfur content has recently been proposed. Lean burn engines and direct injection engines have high energy efficiency and are considered to contribute to reduced carbon dioxide emission. However, because such engines carry out combustion in a high air-fuel ratio range, NOx generation is increased and conventional exhaust gas purification catalysts do not function effectively. It has therefore been attempted to apply NOx storage catalysts as exhaust gas purification catalysts for engines, and according to Toyota Technical Review Vol. 50, No 2, p. 28-33 (December 2000), a finished gasoline sulfur content of no greater than 8 ppm by weight is within the permissible range for catalyst inactivation, suggesting potential application of NOx storage catalysts. The aforementioned conventional gasoline hydrodesulfurization technologies give consistent indications regarding hydrodesulfurization of catalytically-cracked gasoline, but it has not been possible to reach a level that can provide finished gasoline with an extremely low sulfur content of no greater than 8 ppm by weight. Non-patent document 1, identified below, tangentially refers to results of hydrodesulfurization to a sulfur content of 8 ppm by weight, but decrease of the road octane value (the average of the research octane value and motor octane value) is 3.8 compared to before hydrodesulfurization treatment, and therefore the technique cannot be considered practical.
In order to achieve a sulfur content of no greater than 8 ppm by weight for finished gasoline it is necessary to reduce the sulfur content of the catalytically-cracked gasoline, as its compositional base, to no greater than about 10 ppm by weight, and development of such production techniques is expected to be a key technology for production and provision of sulfur-free gasoline.    [Patent document 1] U.S. Pat. No. 4,990,242    [Patent document 2] Japanese Patent Public Inspection No. 2000-505358    [Patent document 3] U.S. Pat. No. 5,352,354    [Patent document 4] U.S. Pat. No. 4,149,965    [Patent document 5] Japanese Unexamined Patent Publication No. 2000-239668    [Non-patent document 1] NPRA Annual Meeting, AM-00-11(2000)